[ Beneath the Waves ]

Thermal versus Near Infrared

article and photographs by Ben Lincoln

 

Perhaps the most common misunderstanding with regards to multispectral photography concerns the imaging of infrared radiation. Most of us (myself included) learn in school that "infrared" and "heat" are the same thing. Then when we learn about "infrared photography", we naturally assume that this is the same thing as thermal imaging (especially when thermal imaging system manufacturers refer to their products as "infrared cameras", which is true if misleading without qualifying what type of infrared is being imaged).

Refer for a moment to the electromagnetic spectrum chart from the beginning of A Detailed Introduction. "The infrared" is actually three distinct bands, the combination of which is much larger than the entire human-visible spectrum.

When photographers refer to "infrared", they are describing the near infrared - the part closest to red light - generally between about 1000nm and 750nm in wavelength. This type of infrared energy behaves almost exactly like human-visible light. It can be focused using glass lenses, and the sensors used in digital cameras can easily detect it[1]. Film cameras can be used for this type of photography as well, provided that special infrared-sensitive film is used. The near infrared is also where conventional night vision systems and infrared remote controls operate[2].

Thermal imaging equipment generally operates in the far infrared region - waves as much as 15 times longer than near infrared light - although some types make use of mid infrared (or both far and mid infrared). Not only are specialized sensors needed, but the lenses of these cameras must be made of exotic glass (created from elements like germanium instead of the silicon which makes up everyday glass). These lenses are completely opaque to visible light, and cannot be used for standard photography.

As it is a specialized application, thermal imaging gear is very expensive compared to conventional cameras, and is generally well beyond the reach of all but the most deep-pocketed hobbyists. Recent advances in technology have cut the prices down to about a quarter of what they were a decade or two ago, but even entry-level thermal imagers start at about US$4000[3]. Although this is about the same price a professional photographer would pay for a high-end DSLR body, these devices are much lower resolution - generally producing an image of 80x60, 160x120, or 320x240 pixels, which is less than even the cheapest bargain-basement webcam delivers in the visible spectrum. FLIR Systems produces several relatively high-resolution models intended for advanced scientific and military use, but these are of the "call for pricing" rarefied-air budget class.

Because near infrared versus thermal infrared is such a frequent point of confusion, here are a couple of images to hammer home the point. First, a set of photos I took of my soldering iron while it was cold:

Soldering Iron at Room Temperature
[ R-G-B ]
R-G-B
[ NIR-Grey ]
NIR-Grey
[ UVA-Grey ]
UVA-Grey
   

This shot doesn't show any interesting variation in false colour, so I've only included the visible light and greyscale NIR/UVA versions.

Date Shot: 2009-09-09
Camera Body: Nikon D70 (Modified)
Lens: Nikon Series E 100mm
Filters: Standard Set
Date Processed: 2009-09-09
Version: 1.0

 

Next, the same soldering iron after it has heated to operating temperature, as it appears in the near infrared:

Soldering Iron at Operating Temperature
[ NIR-Grey ]
NIR-Grey
       

Look, it's hot, there's a wisp of smoke coming off the solder I put on the tip.

Date Shot: 2009-09-09
Camera Body: Nikon D70 (Modified)
Lens: Nikon Series E 100mm
Filters: Standard Set
Date Processed: 2009-09-09
Version: 1.0

 

It's hot enough to melt solder, but there is no trace of a glow visible in the near infrared. Clearly something more is required to "see heat".

Here is a similar soldering iron warming up as captured using an actual thermal imager:

Soldering Iron at Operating Temperature - Thermal IR
[   ]
 
       

The appearance of a hot soldering iron using thermal IR is quite different from near infrared.

Date Shot: 2014-08-14
Camera Body: FLIR E4 (Modified)
Lens: FLIR E4
Filters: None
Date Processed: 2014-08-14
Version: 1.0

 

Hold on though. There is a catch, and I think this catch may have helped cause the confusion in the first place. Although thermal emissions are entirely contained in the mid and far infrared, the near infrared can be used to detect one particular type of hot object: those that are almost hot enough to begin glowing visibly through incandescence. Just as very hot metal (or rock) will glow red, then yellow, then white as its heat increases, it will begin emitting near infrared light even before it begins to glow red.

Here is a closeup of one of the heating elements on my electric stove, with the power turned off:

Stovetop Bokeh (Stove Off)
[ R-G-B ]
R-G-B
[ NIR-Grey ]
NIR-Grey
[ UVA-Grey ]
UVA-Grey
[ NIR-R-G ]
NIR-R-G
[ G-B-UVA ]
G-B-UVA

 

Date Shot: 2009-09-09
Camera Body: Nikon D70 (Modified)
Lens: Nikon Series E 100mm on an extension ring
Filters: Standard Set
Date Processed: 2009-09-09
Version: 1.0

 

Here is the same element at low heat in the near infrared. You can see that it is still just as dark as with no power applied.

Stovetop Bokeh (Stove on Low)
[ NIR-Grey ]
NIR-Grey
       

 

Date Shot: 2009-09-09
Camera Body: Nikon D70 (Modified)
Lens: Nikon Series E 100mm on an extension ring
Filters: Standard Set
Date Processed: 2009-09-09
Version: 1.0

 

The same element at medium heat still is not emitting any near infrared light, even though it has visibly expanded from the higher temperature, and the heat being emitting can easily be felt from 20-30cm away.

Stovetop Bokeh (Stove on Medium)
[ NIR-Grey ]
NIR-Grey
       

 

Date Shot: 2009-09-09
Camera Body: Nikon D70 (Modified)
Lens: Nikon Series E 100mm on an extension ring
Filters: Standard Set
Date Processed: 2009-09-09
Version: 1.0

 

At the "medium high" setting, finally a change emerges in the near infrared, even though the element is still dark to human eyes. The change is easiest to see in the NIR-R-G false colour rendition.

Stovetop Bokeh (Stove on Medium-High)
[ R-G-B ]
R-G-B
[ NIR-Grey ]
NIR-Grey
[ UVA-Grey ]
UVA-Grey
[ NIR-R-G ]
NIR-R-G
[ G-B-UVA ]
G-B-UVA

 

Date Shot: 2009-09-09
Camera Body: Nikon D70 (Modified)
Lens: Nikon Series E 100mm on an extension ring
Filters: Standard Set
Date Processed: 2009-09-09
Version: 1.0

 

Halfway in-between "medium high" and high, the element still is not emitting any human-visible light, but there is no mistaking that a significant near infrared glow is present.

Stovetop Bokeh (Medium-High Plus)
[ R-G-B ]
R-G-B
[ NIR-Grey ]
NIR-Grey
[ UVA-Grey ]
UVA-Grey
[ NIR-R-G ]
NIR-R-G
[ G-B-UVA ]
G-B-UVA

This is with the stove halfway in-between the "Medium-High" and "High" marks.

Date Shot: 2009-09-09
Camera Body: Nikon D70 (Modified)
Lens: Nikon Series E 100mm on an extension ring
Filters: Standard Set
Date Processed: 2009-09-09
Version: 1.0

 

At its highest setting, the element is begining to glow a dull red colour, but is putting out a stunning amount of near infrared light.

Stovetop Bokeh (Stove on High)
[ R-G-B ]
R-G-B
[ NIR-Grey ]
NIR-Grey
[ UVA-Grey ]
UVA-Grey
[ NIR-R-G ]
NIR-R-G
[ G-B-UVA ]
G-B-UVA
[ B&W 403 Only ]
B&W 403 Only
[ B&W 403 and LDP CC1 Stack ]
B&W 403 and LDP CC1 Stack
     

I've included two alternate versions of this shot to make a point about ultraviolet bandpass filters. The first is the result when just a B&W 403 filter is used, and the second is with an LDP CC1 IR-blocking filter stacked on top of it. You can see that without the IR filter, the result is not an ultraviolet image at all, but near infrared. Even with the IR filter, there is still some NIR leakage as evidenced by the glow of the stove element in that version. Compare this to the "regular" UVA version (shot using a Baader U-Filter), where the element is completely dark even though it is blazing away in the NIR.

Date Shot: 2009-09-09
Camera Body: Nikon D70 (Modified)
Lens: Nikon Series E 100mm on an extension ring
Filters: Standard Set plus B&W 403
Date Processed: 2009-09-09
Version: 1.0

 

This last set reinforces my belief that false colour is at least generally preferable to discreet greyscale images. The greyscale near infrared version indicates to the viewer that something is very bright. The false colour NIR-R-G variation intuitively depicts that the stove element is very hot. If you were wearing a set of multispectral goggles, which would you rather see before deciding whether to touch the element or not?

Just to be clear, this is not the same as thermal imaging. It is the everyday phenomenon that people have seen for thousands of years in places like blacksmiths' forges, foundries, and lava flows, but slightly lower on the electromagnetic spectrum.

 
Footnotes
1. See Cameras.
2. See Uses of Multispectral Photography
3. I have seen older secondhand thermal imagers sell for "only" about a thousand dollars, but this is still far too expensive for my hobby budget. If you want to go this route, keep in mind that older devices often require active cooling (which newer designs do not), and may not offer anything other than analogue video output.
 
[ Page Icon ]